1. Field of the Invention
The present invention relates to an optical disc cover layer formation method and an optical disc cover layer formation device, and more specifically to an optical disc cover layer formation method and an optical disc cover layer formation device forming a cover layer by applying a photo-curing-type resin in liquid form to a disc substrate by a spin-coating method and irradiating light of a predetermined wavelength at the photo-curing-type resin for curing.
2. Description of the Related Art
As optical discs which replay or record information using laser light, CDs (compact disc), CD-Rs (compact disc-recordable), DVDs (digital versatile disc), DVD-Rs (digital versatile disc-recordable) and the like are becoming widespread. In recent years, optical discs have been proposed whose capacities are made larger in comparison to conventional CDs, DVDs and the like by using short-wavelength blue-violet laser lights for laser light and increasing numerical aperture (NA) of object lenses up to 0.85.
Because a focusing distance of the laser light is short with these large capacity optical discs, it is necessary to form a cover layer (laser light transmission layer), which protects an information recording layer provided at a top face (a plate surface) of a disc substrate, more thinly and with higher accuracy of thickness (for example, with a thickness dimension of around 0.1 mm, tolerance is of the order of ±3 μm).
Conventionally, such a cover layer has been formed by supplying a liquid-form resin, which is a source material for the cover layer, to a central portion vicinity of a disc substrate, applying the resin to the disc substrate by a spin-coating method, of rotating the disc substrate and spreading the resin by centrifugal force, and thereafter applying a predetermined curing process of heating the resin in the case of a heat curing-type resin, irradiating UV light in the case of an UV (ultraviolet) curing-type resin, or the like. However, with such a spin-coating method, differences in coating thickness may arise between a center side and a peripheral edge side of the disc surface, depending on a balance between viscosity (flowability) and spreading amounts (coating amounts) of the resin, which is spread in the diametric direction of the disc substrate by centrifugal force due to the spinning of the disc substrate, and the like. For example, there is a tendency for resin to be thicker at the center side of a disc surface to which the resin is supplied, becoming thinner toward the peripheral edge side in accordance with the spreading.
Accordingly, for the large capacity optical discs for which higher accuracy of thickness of the cover layer is required as mentioned above, many techniques for suppressing irregularities in resin thickness have been proposed. For example, a technique has been proposed which suppresses unevenness in thickness of a resin and raises accuracy of thickness of a cover layer by: setting a round plate portion of closing off means at a central hole of a disc substrate to close a central hole; while rotating the disc substrate at low speed, supplying liquid-form resin to a support shaft, which is standingly provided at an axial center portion of the round plate portion and rotates with the disc substrate, to distribute the resin uniformly on the round plate portion; and thereafter rotating the disc substrate at high speed to spin-coat the resin (see, for example, FIGS. 3 and 4 of Japanese Patent Application Laid-Open (JP-A) No. 2001-351275).
However, with the spin-coating method described in the patent reference mentioned above, the resin, which is uniformly applied to the round plate portion of the closing off means beforehand, is caused to flow without alteration from the round plate portion to the disc substrate by spin coating. Hence, irregularities in thickness are suppressed. However, this method is the same as convention in that the resin on the disc substrate, where the upper face is open, is spread only by centrifugal force. Consequently, as might be expected, there is a problem in that, in order to raise the accuracy of coating thickness, specification of conditions such as the viscosity of the resin, the rotation speed and rotation duration of the disc substrate and the like is complicated. Further, there is a problem in that production steps have to be strictly controlled.